The present invention relates in a general way to an X-ray filter for shaping the beam of rays and compensating for the differences in X-ray absorption by a region under examination of a body having areas of different absorption densities and thus avoiding overexposure of the image obtained in the areas of the image corresponding to the areas of low absorption densities. The invention also relates to X-ray imaging machines incorporating such a filter, in particular medical imaging machines.
Digital X-ray imaging machines used for vascular or cardiac imaging are generally provided with field-shaping (FS) filters inserted into the path of the X-ray beam, between the X-ray source and the region under examination of a patient's body, in order to avoid overexposure of the contour of the image obtained due to saturation of the video camera of the machine. This is because, in some cases, the region under examination of a patient's body may have very dense areas contiguous with low-density areas. This is the case, for example, in pulmonary vascular examinations in which the areas of the spinal column and heart are very dense compared to the area of the lungs. The insertion of a filter made of an X-ray-absorbent material opposite the low-density areas makes it possible to equalize the image contrast in the areas of the image corresponding to the low-density areas of the region examined.
Referring to FIG. 1, which shows diagrammatically the principle of operation of a shaping filter, an X-ray beam passes through a region 1 of a patient's body, which includes an area 2 of low absorption density, and is then collected by an image intensifier whose output signals are processed in the imaging machine (not shown) in order to obtain an image of the region 1 examined. A shaping filter 5, placed in the X-ray beam 3 between the X-ray source and the region 1 examined, has a central opening 6 defining the field of view of the image obtained. In order to avoid overexposure of the image in that area of the latter corresponding to the low-density area 2 of the region 1 under examination, a movable thin plate of the filter 5, made of X-ray-absorbent material, is moved by the operator in such a way that this plate covers an area of the central opening 6 of the filter corresponding to the low-density area 2 of the region 1 examined. Thus, overexposure of the image in this area is avoided because of the absorption of the X-rays by the thin plate which compensates for the low absorption by the low-density area 2.
Depicted in FIG. 2 is a shaping filter 10 conventionally used in X-ray imaging machines. The shaping filter 10 comprises a main frame 11 in the form of a flat ring having a central circular opening 12 for the passage of the X-ray beam. Two parallel straight sliding rails 13, 14 are fixed to one of the main surfaces of the main frame 11 in diametrically opposed positions.
Two curved compensating plates 17, 18, made of X-ray-absorbent material, having the general shape of crescents whose curvature corresponds to that of the central opening 12, are joined by one of their ends by means of a carriage (not depicted), each respectively, to one of the sliding rails 13, 14 so as to be able to be moved over the main frame 11, one plate over the other, by translation along their respective rail, between a retracted position in which the compensating plates 17, 18 lie almost entirely over the main frame 11 and active positions in which the plates 17, 18 are over the central opening 12.
As shown in FIG. 2, the compensating plates 17, 18 are placed symmetrically with respect to the center 15 of the main frame 11.
When the plates of the compensation 17, 18 are in their retracted positions, the internal edges of the plates 17, 18 define the maximum field of view 19 of the X-ray image and when they are in active positions they define the effective field of the X-ray image. Thus, by adjusting the position of the plates 17, 18 by translation along the rails 13, 14, the contour of the field of view is defined and the differences in absorption can be compensated for.
The entire filter 10 can rotate about the center 15 in order to comply with the orientation of the region examined.
It would be desirable to be able to use standard shaping filters, like the one in FIG. 2, in order to define the field of view and obtain the desired absorption compensation, but the mechanism for moving the plates 17, 18 of these filters limits the maximum width (x) of the plates to the width (y) between the maximum field of view 19 and the outer edge of the main frame 11. Unfortunately, the wide low-density areas of a patient (for example, the lungs) exceed the capabilities of the conventional shaping filters, such as those in FIG. 2. It would be possible to design an entirely new mechanism, but this solution would be expensive and would increase the volume of the X-ray-beam collimator in unacceptable proportions.